Device for scanning a disc-shaped record carrier

Description

[0001] The invention relates to a device for scanning a disc-shaped record carrier having a track in the form of a spiral which substantially surrounds a point of rotation of the record carrier, said device comprising a record carrier driving device for causing the record carrier to rotate about the point of rotation, a scanning head support which is radially displaceable with respect to the record carrier, a scanning head support driving device for displacing the scanning head support, said scanning head support having a scanning head for scanning the record carrier with a scanning point, the device further comprising a control system for keeping the scanning point on the track, said control system comprising a fine-positioning device arranged on the scanning head support for radially displacing the scanning point across the record carrier within a predetermined limited range of displacement by varying a position of the fine-positioning device with respect to the scanning head support, means for supplying a position-indication signal which is indicative of the position of the fine-positioning device, means for detecting whether the position-indication signal exceeds a predetermined threshold value, and control means for temporarily energizing the scanning head support driving device in response to the detection of exceeding the threshold value in order to realise a displacement of the scanning head support towards a position at which the position-indication signal resumes a value below the exceeded threshold value, the control means being adapted to energize the scanning head support driving device in conformity with an energization pattern which is independent of the instantaneous value of the position-indication signal.

[0002] Such a device is known from GB-B-1,429,882. This document describes a reading device for optical record carriers. In this reading device an optical scanning head is used with which a focused light beam is directed onto the record carrier, with a scanning spot being formed on the record carrier at the location of the scanning point. This scanning spot can be radially displaced across the record carrier by means of the fine-positioning device in the form of a pivotable mirror. The scanning head including the pivotable mirror is mounted on a slide which can be radially displaced by means of the driving device. This driving device comprises a motor-driven spindle which cooperates with a rack provided on the slide.

[0003] The radial position of the slide is continuously corrected in dependence upon the position-indication signal so that the pivotable mirror substantially remains proximate to the centre of the range of displacement. Furthermore, said Patent Specification states that it is possible to activate the driving device for the slide only when the excursion of the pivotable mirror has exceeded an extreme position.

[0004] In the known device the control of the slide position comprises a feedback. This feedback yields problems if the transmission ratio of the spindle/rack combination comprised by the driving device is to be increased in order to increase the access speed when searching information on the record carrier. Due to hysteresis in the driving system the displacement of the scanning head will often exhibit an unwanted oscillatory character at large transmission ratios. These oscillations have a detrimental influence on the fine-positioning of the scanning point. Moreover, these oscillations result in a higher energy consumption. These oscillations may also result in an audible production of sound and extra wear.

[0005] It is, inter alia, an object of the invention to provide a device as described in the opening paragraph in which the above-mentioned drawbacks are obviated.

[0006] According to the invention, this object is achieved by means of a device as described in the opening paragraph, which is characterized in that the control means comprise detection means for detecting the resulting effect of the energization pattern on the position-indication signal and setting means for adapting, after the energization, a setting of said threshold or said energization pattern in dependence upon the detected effect.

[0007] In the device according to the invention there is no feedback of the instantaneous value of the position-indication signal to the control of the driving device for the scanning head support so that oscillations in the positioning of the scanning head are prevented.

[0008] In principle, it is possible to set the control means once, for example, during or immediately after assembly of the device and to leave this setting always unmodified. However, the circumstances in which the driving device is to operate may vary considerably during use so that the control setting is not always adapted to these circumstances. This drawback also is obviated by the invention.

[0009] A further embodiment of the device is characterized in that the control means are adapted to determine the influence of the energization in a predetermined time interval after terminating the energization.

[0010] By postponing the determination of the influence of the energization, it is achieved that the scanning head support has already come to a standstill at the instant of determination so that a great accuracy of determining the influence is obtained. As a result of an eccentric location of the point of rotation, the position of the fine-positioning device will not change monotonically, but the fine-positioning device will reciprocate around a slowly varying average position. In the case of a large eccentricity it may occur that, due to the eccentricity, there is a movement towards the end of the range after the driving device has been energized.

[0011] An embodiment of the device which obviates this drawback is characterized in that the control means for determining the influence of the energization comprise means for detecting whether a second threshold value having a sign which is opposite to the first threshold value is exceeded by an absolute value which is larger than that of the first threshold value, the control means being adapted to energize the scanning head support driving device in conformity with a second energization pattern which results in a displacement of the scanning head in a direction opposite to the displacement caused by the first-mentioned energization pattern.

[0012] The invention is eminently suitable for use in combination with scanning head driving devices of the self-braking type, such as, for example driving devices using a spindle-rack combination. In that case the scanning head is kept in position between two energization operations and without the supply of energy from the exterior so that the positioning of the scanning head support is substantially insensitive to exterior shock-like disturbances.

[0013] However, the invention is not limited to devices in which self-braking driving devices are used. Other driving devices using, for example a linear motor are alternatively possible. In the latter case it is recommendable to actively brake the linear motor between two energization operations.

[0014] The invention will hereinafter be described in greater detail with reference to Figs. 1 to 7 in which

Fig. 1 is a plan view of a disc-shaped record carrier having a spiral track,

Fig. 2 shows an embodiment of a scanning device according to the invention,

Fig. 3 shows an embodiment of a control circuit for use in the scanning device,

Figs. 4 and 8 show the variation of the position-indication signal and

Figs. 5, 6 and 7 show flow charts of programs performed by a control unit of the scanning device.

[0015] Fig. 1 shows a disc-shaped record carrier 1 of a conventional type, for example a magnetic or optical record carrier. The record carrier 1 has a spiral track 2 which surrounds a point of rotation 3.

[0016] Fig. 2 shows an embodiment of a device for scanning the track 2 on the record carrier 1. The device comprises a driving device in the form of a motor 4 for causing the record carrier 1 to rotate about the point of rotation 3. A scanning head 5 of a conventional type, for example an optical or magnetic read and/or write head is arranged opposite the rotating record carrier 1. The scanning head scans the record carrier 1 at the location of a scanning point 6. The scanning head 5 is secured to a scanning head support, for example in the form of a slide 7 which is radially displaceable with respect to the record carrier. The scanning point can be radially displaced with respect to the slide 7 by means of a fine-positioning device 8 of a conventional type. The scanning point 6 is kept directed on the track by means of a control system of a conventional type. In the embodiment shown the control system comprises the fine-positioning device 8 and a control circuit 9. The control circuit 9 derives a tracking error signal from detection signals supplied by the scanning head 5 and derives a control signal for the fine-positioning device 8 from the tracking error signal, such that the tracking error signal maintains a value which is substantially equal to zero. The scanning device further comprises a driving device for radially displacing the slide 7. The driving device is preferably of the self-braking type, i.e. it is not necessary to supply energy from the exterior to retain the slide in position in its non-energized state. Driving devices of the self-braking type have the advantage that they are not very sensitive to shock-like disturbances from the exterior, also when they are in a non-energized state. However, it is to be noted that the driving device may alternatively be of a type different from the self-braking type. For example, a driving device comprising a linear motor may also be used. To render such a driving device insensitive to shock-like disturbances, it is recommendable to provide the driving device of a non-selfbraking type with an active brake. In the embodiment shown the driving device is provided with a lead screw nut secured to the slide, which nut cooperates with a spindle 10 driven by a motor 11.

[0017] In the scanning device of Fig. 2 a position-indication signal PS is generated, which signal is indicative of the position of the fine-positioning device 8 with respect to the slide 7. If the fine-positioning device occupies a position which is substantially proportional to the received control signal and the control circuit 9 comprises a control means which is provided with an integrator for realising an integrating control action, the output signal of this integrator may serve as the position-indication signal PS. However, it is alternatively possible to use a separate position detector for obtaining the position-indication signal PS. The use of the output signal of the integrator of the control circuit is, however, preferred because no extra components are then required.

[0018] For the purpose of illustration Fig. 3 shows a control circuit of a conventional type from which the position-indication signal can be derived. In this Figure reference numeral 30 denotes a circuit of a conventional type for deriving a tracking error signal RE from the detection signals Id supplied by the scanning head. The tracking error signal is applied to a proportional amplifier 31 for obtaining a proportional control action, an integrator 32 for obtaining an integrating control action and a differentiator 33 for obtaining a differentiating control action. The output signals of the proportional amplifier 31, the integrator 32 and the differentiator 33 are combined in an adder circuit to a control signal Vs for the fine-positioning device 8. The position-indication signal can be derived from the output of the integrator 32.

[0019] The device shown in Fig. 1 further comprises a control unit which, on the basis of the position-indication signal PS, derives an energization signal Vb for the driving motor 11 for displacing the slide 7. The derivation of the energization signal Vb on the basis of the position-indication signal PS will be elucidated with reference to Fig. 4. In this Figure the position-indication signal is shown as a function of time t. At instant t0 the position-indication signal PS has a value PS1 which is slightly lower than the value M corresponding to the central position of the fine-positioning device. The energization signal Vb for driving motor 11 is equal to zero, which means that the position of the slide remains unchanged. While the track is being scanned, the scanning point is held on the track 2. Due to the fact that the track 2 is spiral shaped, the position of the fine-positioning device will slowly change. At instant t1 the fine-positioning device reaches its central position. At instant t2 the signal PS reaches a value LV1. Exceeding of this value is detected by the control unit 12. In response to this detection the control unit generates an energizing pulse having a given energization pattern. Due to this energization pattern, the slide 7 will be displaced over a given distance by the motor 11. This energization pattern may have various forms. However, the pattern preferably comprises an acceleration pulse 40 and a braking pulse 41 of such a strength that the slide comes to a complete or substantially complete standstill at the end of the braking pulse (at instant t3). While the slide 7 is being displaced, the control unit keeps the scanning point 6 on the track so that during displacement of the slide 7 the fine-positioning device is brought to a different position which is located at the other side of the central position of the fine-positioning device. After displacement of the slide, the position of the fine-positioning device will gradually change again until at instant t4 the position is reached at which the associated position-indication signal reaches the threshold value LV again and an energizing pulse is supplied again by the control unit. In the manner described hereinbefore the slide is thus displaced by way of a pulsatory energization whenever the position fixed by the threshold value is reached. The advantage thereof is that the energy required for displacing the slide 7 is minimal. Moreover, the slide is at a standstill between two energizing pulses so that there is little disturbance of the control of the fine-positioning device due to the position control. This is in contrast to a control in which the position of the slide is continuously corrected on the basis of the instantaneous position of the fine-positioning device. Particularly at high transmission ratios of the driving device of the slide such a control involves the risk that the slide positioning will exhibit an oscillatory behaviour.

[0020] The circumstances in which the positioning of the slide must be performed may considerably vary with time so that the distance covered by the slide 7 as a result of the energizing pulse may vary. For example, the position of the scanning device is of great influence on this distance. In fact, dependent on this position, the gravitational force will promote or inhibit the displacement to a smaller or larger extent.

[0021] It is therefore preferred to adapt the setting of the control unit for generating the energizing pulses, dependent on the influence of the energizing pulse on the displacement of the slide 7.

[0022] For example, it is desirable that the working range of the fine-positioning device is located substantially symmetrical with respect to its central position. This can be achieved by adapting the threshold value LV1 at a constant pattern of the energizing pulse, at which a pulse is to be supplied. This may be realised, for example, by determining the signal value of the position-indication signal after the energizing pulse has been supplied and to compare the absolute value of this signal value with the threshold value LV1. If the given absolute value is smaller than LV1, the value of LV1 is decreased and if the absolute value is larger than the value LV1, the value of LV1 is increased. This method may be extended by increasing the energy contents of the energizing pulses each time in steps until the variation of the signal value of PS caused by the energizing pulse has reached a given minimum value, and by subsequently adapting the threshold value LV1 until the working range of the fine-positioning device is located symmetrically around its central position. Another interesting method of adapting the setting of the control unit 12 is the method in which the energy contents of the energizing pulse are adapted in such a way that the displacement caused by the energizing pulse remains within predetermined limits.

[0023] The control unit 12 may be implemented as a "hardwired" circuit or as a program-controlled control unit which is loaded with a suitable control program. Fig. 5 shows a flow chart of a program by way of example, suitable for adapting the energy contents of the energizing pulse in order to keep the displacement caused by the energizing pulse within predetermined limits. This program comprises a step S1 in which the value of the position-indication signal is read. This step is followed by a step S2 in which a logic variable TRIGGER indicates by means of the value .T. that an energizing pulse has just been supplied and a value .F. that no energizing pulse has been supplied. If the value of the variable TRIGGER is .F., step S3 is performed. In this step it is checked whether the value of PS which has been read is below or above the threshold value LV1. If the read value is above the threshold LV1, step S3 is followed by step S4 in which a program function PULSE(USG) is called for supplying the energizing pulse with an energy content fixed by USG. This function will hereinafter be described in greater detail. After step S4 the logic value .T. is assigned to the variable TRIGGER when performing step S5. Subsequently, a waiting step S6 is performed in which a waiting time is observed for the further execution of the program until a predetermined time T has elapsed.

[0024] If it has appeared in the execution of step S3 that the value PS which has been read is below the threshold value LV1, step S3 is immediately followed by step S6. After step S6, step S7 is carried out in which an auxiliary variable PSO is rendered equal to the last value of PS which has been read. Subsequently the program is continued again with the reading step S1.

[0025] When it is subsequently apparent during execution of step S2 that the variable TRIGGER has the logic value .T., and consequently an energizing pulse has just been supplied, step S8 is carried out. In this step the value of the displacement of the fine-positioning device caused by the energizing pulse is determined by determining the absolute value STEP of the difference between the last read value of PS and the value PSO of the last-but-one read value of PS. In step S9 it is subsequently determined by comparing the value of STEP with a threshold value STMIN whether the realised displacement is smaller than a minimum distance. If positive, the value of the variable of the energy contents of the energizing pulses is increased during the execution of step S10. Subsequently step S11 is carried out in which the variable TRIGGER acquires the logic value .F..

[0026] If it has appeared during the execution of step S9 that the variable STEP is larger than STMIN, it is checked during the execution of step S12 whether the STEP is larger than the threshold value STMAX. If positive, the variable USG fixing the energy contents of the energizing pulse is decreased in step S13. Subsequently step S11 is carried out. If it has appeared during the execution of step S12 that the value of STEP is below the threshold value STMAX, step S12 is directly followed by step S11. After the execution of step S11 the program is continued with the waiting step S6.

[0027] Fig. 7 shows a flow chart of an example of the program function PULSE. After calling the function, step S20 is first performed in which an energizing signal Vb with a signal value USG is applied to the motor 11 so that the motor 11 will accelerate. Subsequently there is a waiting time interval having the length of an acceleration time tacc during the execution of step S21. Subsequently the sign of the energizing signal Vb is reversed during the execution of step S22, so that the motor is slowed down again. Then there is a waiting time covering a delay interval tdec during the execution of waiting step S22. After this time interval has elapsed, the value of the energizing signal is set to zero in step S29 and there is a return to the program in which the program function was called. In the example of the program function described hereinbefore the energy contents of the energizing pulse are adapted by adapting the amplitude of the energizing pulse. It will be evident to those skilled in the art that the energy contents may also be adapted in another manner, for example by adapting one or both time intervals tacc and tdec.

[0028] Fig. 4 shows the variation of the value of the position-indication signal between the energizing pulses in a monotonically increasing function of time t. It is to be noted that this will hardly occur in practice. In fact, the point of rotation 3 is generally located slightly eccentrically with respect to the centre of the spiral track. Consequently, the fine-positioning device will reciprocate around a monotonically varying average position during scanning. This results in a position-indication signal comprising a periodical component. A large periodical component may cause problems as will hereinafter be described with reference to Fig. 8. This Figure shows the time variation of a position-indication signal which has a unusually strong periodical component caused by the eccentricity. As a result of this periodical component the signal PS will intersect the threshold value LV1 already at an instant t5 when the average value (indicated by the broken line 80) of the position signal PS has reached the threshold value. As a result of this intersection an energizing pulse is supplied so that the slide and hence the fine-positioning device are displaced. Due to the eccentricity it may occur that a short time after the energizing pulse the fine-positioning device approaches the end of its range of displacement. Reaching the end of the range of displacement can be prevented by detecting an intersection of a threshold value of LV2 which corresponds to a position which is located proximate to the end of the range of displacement and by subsequently applying, in response to this detection, an energizing pulse to the motor 11 which displaces the slide in a direction opposite to the direction of the previous slide displacement.

[0029] Fig. 6 shows a flow chart of an adapted program with which the fine-positioning device can be prevented, in the way as described hereinbefore, from reaching the end of its range of displacement. The program shows great similarity to the previously described program. It comprises three additional steps S14, S15 and S16. The step S14 is performed if it has appeared during the execution of step S3 that the value of the last position-indication signal PS which has been read is below the threshold value LV1.

[0030] In step S14 it is tested whether the last value which has been read is below the threshold value LV2. If negative, step S14 is followed by waiting step S6. If positive, the fine-positioning device tends to approach the end of its range of displacement. In that case the program function PULSE is called in step S15, in which step an energizing pulse is generated with a sign which is opposite to the sign of the last supplied energizing pulse. Subsequently the logic value .T. is assigned to the variable TRIGGER in step S16. After the execution of step S16 the program is continued again with the waiting step S6.

Claims

1. A device for scanning a disc-shaped record carrier (1) having a track (2) in the form of a spiral which substantially surrounds a point of rotation (3) of the record carrier, said device (4) comprising a record carrier driving device for causing the record carrier to rotate about the point of rotation, a scanning head support (7) which is radially displaceable with respect to the record carrier, a scanning head support driving device (11) for displacing the scanning head support, said scanning head support having a scanning head (5) for scanning the record carrier with a scanning point (6), the device further comprising a control system (9) for keeping the scanning point (6) on the track, said control system (9) comprising a fine-positioning device (8) arranged on the scanning head support for radially displacing the scanning point across the record carrier within a predetermined limited range of displacement by varying a position of the fine-positioning device with respect to the scanning head support, means (32) for supplying a position-indication signal which is indicative of the position of the fine-positioning device, means for detecting whether the position-indication signal exceeds a predetermined threshold value (LV1), and control means (12) for temporarily energizing the scanning head support driving device (11) in response to the detection of exceeding the threshold value in order to realise a displacement of the scanning head support (7) towards a position at which the position-indication signal resumes a value below the exceeded threshold value, the control means being adapted to energize the scanning head support driving device in conformity with a given energization pattern (40)which is independent of the instantaneous value of the position-indication signal, characterized in that the control means comprise detection means for detecting the resulting effect of the energization pattern on the position-indication signal and setting means for adapting, after the energization, a setting of said threshold value (LV1) or said energization pattern (40) in dependence upon the detected effect.

2. A device as claimed in Claim 1, characterized in that the control means (12) are adapted to determine the resulting effect of the energization in a predetermined time interval after terminating the energization.

3. A device as claimed in Claims 1 or 2, characterized in that the control means (12) for determining the resulting effect of the energization comprise means for detecting whether the displacement caused by the energization is located within predetermined limits the setting means being arranged to adapt the energization pattern if the displacement is located outside said limits.

4. A device as claimed in any one of the preceding Claims, characterized in that for determining the resulting effect of the energization the control means comprise means for detecting whether a second threshold value (LV2) having a sign which is opposite to the sign of the first threshold value (LV1) is exceeded by an absolute value which is larger than that of the first threshold value, the control means being adapted to energize the scanning head support driving device (11) in conformity with a second energization pattern which results in a displacement of the scanning head (5) in a direction opposite to the displacement caused by the first-mentioned energization pattern.

5. A device as claimed in any one of the preceding Claims, characterized in that the scanning head support driving device (11) is of a self-braking type.

Ansprüche

1. Einrichtung zum Abtasten eines plattenförmigen Aufzeichnungsträgers (1) mit einer Spur (2) in Form einer Spirale, die im wesentlichen einen Drehpunkt (3) des Aufzeichnungsträgers umgibt, wobei diese Einrichtung (4) eine Aufzeichnungsträgeransteuerungseinrichtung umfaßt, um den Aufzeichnungsträger um den Drehpunkt drehen zu lassen, einen Abtastkopfträger (7), der in bezug auf den Aufzeichnungsträger radial verlagerbar ist, eine Abtastkopfträgeransteuerungseinrichtung (11) zum Verlagern des Abtastkopfträgers, wobei der Abtastkopfträger einen Abtastkopf (5) zum Abtasten des Aufzeichnungsträgers mit einem Abtastpunkt (6) hat, wobei die Einrichtung weiterhin ein Steuerungssystem (9) umfaßt, um den Abtastpunkt (6) auf der Spur zu halten, wobei das genannte Steuerungssystem (9) eine auf dem Abtastkopfträger angeordnete Feinpositionierungseinrichtung (8) umfaßt, zum radialen Verlagern des Abtastpunktes über dem Aufzeichnungsträger in einem zuvor bestimmten begrenzten Verlagerungsbereich durch Veränderung einer Position der Feinpositionierungseinrichtung in bezug auf den Abtastkopfträger, sowie Mittel (32) zum Abgeben eines Positionsanzeigesignals, das die Position der Feinpositionierungseinrichtung anzeigt, Mittel zum Detektieren, ob das Positionsanzeigesignal einen zuvor bestimmten Schwellenwert (LV1) überschreitet und Steuerungsmittel (12) zum vorübergehenden Erregen der Abtastkopfträgeransteuerungseinrichtung (11) in Reaktion auf die Detektion der Überschreitung des Schwellenwertes, um eine Verlagerung des Abtastkopfträgers (7) hin zu einer Position, bei der das Positionsanzeigesignal einen Wert unterhalb des überschrittenen Schwellenwertes annimmt, zu bewirken, wobei die Steuerungsmittel eingerichtet sind, die Abtastkopfträgeransteuerungseinrichtung entsprechend einem gegebenen Erregungsmuster (40) zu erregen, das unabhängig vom momentanen Wert des Positionsanzeigesignals ist, dadurch gekennzeichnet, daß die Steuerungsmittel Detektionsmittel zum Detektieren des resultierenden Effektes des Erregungsmusters auf das Positionsanzeigesignal sowie Einstellmittel zum Anpassen, nach der Erregung, einer Einstellung des genannten Schwellenwertes (LV1) oder des genannten Erregungsmusters in Abhängigkeit vom detektierten Effekt umfassen.

2. Einrichtung nach Anspruch 1, dadurch gekennzeichnet, daß die Steuerungsmittel (12) eingerichtet sind, den resultierenden Effekt der Erregung in einem zuvor bestimmten Zeitintervall nach Beendigung der Erregung zu bestimmen.

3. Einrichtung nach einem der Ansprüche 1 oder 2, dadurch gekennzeichnet, daß die Steuerungsmittel (12) zum Bestimmen des resultierenden Effektes der Erregung Mittel umfassen zum Detektieren, ob die von der Erregung bewirkte Verlagerung innerhalb zuvor bestimmter Grenzen liegt, wobei die Einstellmittel zum Anpassen des Erregungsmusters ausgebildet sind, wenn die Verlagerung außerhalb der genannten Grenzen liegt.

4. Einrichtung nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß zum Bestimmen des resultierenden Effektes der Erregung die Steuerungsmittel Mittel umfassen zum Detektieren, ob ein zweiter Schwellenwert (LV2) mit einem Vorzeichen, das dem Vorzeichen des ersten Schwellenwertes (LV1) entgegengesetzt ist, durch einen Absolutwert überschritten wird, der größer ist als der des ersten Schwellenwertes, wobei die Steuerungsmittel ausgebildet sind, die Abtastkopfträgeransteuerungseinrichtung (11) entsprechend einem zweiten Erregungsmuster zu erregen, was zu einer Verlagerung des Abtastkopfes (5) in einer der vom erstgenannten Erregungsmuster bewirkten Verlagerung entgegengesetzten Richtung führt.

5. Einrichtung nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß die Abtastkopfträgeransteuerungseinrichtung (11) von einer selbstbremsenden Art ist.

Revendications

1. Dispositif pour balayer un support d'enregistrement en forme de disque (1) pourvu d'une piste (2) ayant la forme d'une spirale qui entoure sensiblement un point de rotation (3) du support d'enregistrement, ledit dispositif (4) comprenant un dispositif d'entraînement du support d'enregistrement pour entraîner le support d'enregistrement en rotation autour du point de rotation, un support de tête de balayage (7) qui peut être déplacé radialement par rapport au support d'enregistrement, un dispositif d'entraînement de support de tête de balayage (11) pour déplacer le support de tête de balayage, ledit support de tête de balayage étant pourvu d'une tête de balayage (5) pour balayer le support d'enregistrement avec un point de balayage (6), le dispositif comprenant, en outre, un système de commande (9) pour maintenir le point de balayage (6) sur la piste, ledit système de commande (9) comprenant un dispositif de positionnement fin (8) disposé sur le support de tête de balayage pour déplacer radialement le point de balayage en travers du support d'enregistrement dans une gamme de déplacement limitée prédéterminée en faisant varier une position du dispositif de positionnement fin par rapport au support de tête de balayage, des moyens (32) pour délivrer un signal d'indication de position qui indique la position du dispositif de positionnement fin, des moyens pour détecter si le signal d'indication de position dépasse une valeur de seuil prédéterminée (LV1), et des moyens de commande (12) pour activer temporairement le dispositif d'entraînement de support de tête de balayage (11) en réaction à la détection d'un dépassement de la valeur de seuil afin de réaliser un déplacement du support de tête de balayage (7) vers une position dans laquelle le signal d'indication de position reprend une valeur inférieure à la valeur de seuil dépassée, les moyens de commande étant à même d'alimenter le dispositif d'entraînement de support de tête de balayage en conformité avec un motif d'activation donné (40) qui est indépendant de la valeur instantanée du signal d'indication de position, caractérisé en ce que les moyens de commande comprennent des moyens de détection pour détecter l'effet résultant du motif d'activation sur le signal d'indication de position et des moyens de réglage pour adapter, après l'activation, un réglage de ladite valeur de seuil (LV1) ou dudit motif d'activation en fonction de l'effet détecté.

2. Dispositif suivant la revendication 1, caractérisé en ce que les moyens de commande (12) sont à même de déterminer l'effet résultant de l'activation dans un intervalle de temps prédéterminé après que l'activation a pris fin.

3. Dispositif suivant la revendication 1 ou 2, caractérisé en ce que les moyens de commande (12) destinés à déterminer l'effet résultant de l'activation comprennent des moyens pour détecter si le déplacement provoqué par l'activation se trouve dans les limites prédéterminées, les moyens de réglage étant à même d'adapter le motif d'activation si le déplacement sort desdites limites.

4. Dispositif suivant l'une quelconque des revendications précédentes, caractérisé en ce que pour déterminer l'effet résultant de l'activation, les moyens de commande comprennent des moyens pour détecter si une deuxième valeur de seuil (LV2) de signe opposé au signe de la première valeur de seuil (LV1) est dépassée par une valeur absolue qui est plus grande que celle de la première valeur de seuil, les moyens de commande étant à même d'alimenter le dispositif d'entraînement de support de tête de balayage (11) en conformité avec un deuxième motif d'activation qui produit un déplacement de la tête de balayage (5) dans une direction opposée au déplacement produit par le premier motif d'activation mentionné.

5. Dispositif suivant l'une quelconque des revendications précédentes, caractérisé en ce que le dispositif d'entraînement de support de tête de balayage (11) est d'un type autofreinant.

Drawing